In our parent U.S. application Ser. No. 409,838 U.S. Pat. No. 4487743 controlled low expansion, nickel-iron and nickel-cobalt-iron alloys are described and claimed, the alloys being characterized by (i) an inflection temperature of at least 625.degree. F., (ii) a coefficient of expansion between ambient and inflection temperature not greater than 5.5 .times.10.sup.-6 per .degree. F., (iii) high room temperature tensile strength, (iv) improved elevated temperature stress-rupture properties, including notch-rupture strength, (v) good notch ductility (notch bar rupture life exceeds smooth bar rupture life), etc.
The alloys as set forth in U.S. Ser. No. 409,838 U.S. Pat. No. 4487743 contain about 34% to 55% nickel, up to 25% cobalt, about 1% to 2% titanium, about 1.5% to 5.5% columbium, about 0.25% to 1% silicon, not more than about 0.2% aluminum, not more than about 0.1% carbon, with iron being essentially the balance. A more advantageous and preferred composition contains about 35% to 39% nickel, about 12% to 16% cobalt, about 1.2% to 1.8% titanium, about 4.3% to 5.2% columbium, about 0.3% to 0.5% silicon, not more than about 0.1% aluminum, not more than about 0.1% carbon, with iron again constituting essentially the balance.
A number of Heat Treatments as applied to the alloys above-described were also set forth as follows
Heat Treatment "A": anneal at 1700.degree. F./1hr; AC; age at 1325.degree. F./8hr; FC to 1150.degree. F. at 100.degree. F./hr; age at 1150.degree. F./8hr; AC PA0 Heat Treatment "B": same as "A" except anneal at 1800.degree. F. PA0 Heat Treatment "C": same as "A" except anneal at 1900.degree. F. PA0 Heat Treatment "D": same as "B" except first aging at 1425.degree. F. PA0 Heat Treatment "E": same as "C" except first aging at 1425.degree. F. PA0 Heat Treatment "F": same as "A" except first aging at 1425.degree. F. PA0 Heat Treatment "G": same as "A" except first cooling step is a WQ PA0 Heat Treatment "H": same as "C" except first aging at 1425.degree. F. for 24 hrs. PA0 K.sub.t =2 Notch Bar PA0 K.sub.t =2 Notch Bar
Note: AC=air cool; FC=furnace cool; WQ=water quench PA1 i. only one notch bar failed in the notch section, all other tests having been discontinued or failed in smooth bar PA1 ii. the notch tests at 130 ksi were discontinued after 1000 hrs PA1 iii. of the notch tests at 145 ksi, one fractured (1325.degree. F. age) in the notch at approximately 100 hrs life PA1 iv. tests given higher aging temperatures broke in the smooth ligament PA1 i. rupture strength decreased with increasing aging temperature however, PA1 ii. rupture ductility increased PA1 i. a comparison between smooth bar and K.sub.t =2 notch bar life indicates that only the 1325.degree. F. age evidenced signs of notch brittleness PA1 ii. the notch bar to smooth bar rupture life ratio markedly increased at aging temperatures above 1325.degree. F. PA1 notch bar life at 1000.degree. F./120 ksi increased as aging temperature was raised
The foregoing heat treatments utilized relatively extended periods of time. A basic purpose of the instant invention was to reduce processing time.